For now, the server codebase doesn't really know if we are using a parent process only or a multiprocess browser toolbox/console.

Having an explicit field to distinguish the two would help implement a third mode where we focus only on the current tab's process (bug 1772310), but also enabling EFT on the browser toolbox (bug 1785266).

For the latter, this will actually help land the incremental refactoring in bug 1785106 by spawning only the parent process target actor from the server. And then bug 1785266 to tweak the browsing context filtering to only accept parent process browsing context.

This new data is passed down to all function filtering in/out the WindowGlobal's.
This will be used in a next patch to:

• allow listening for frame targets in the parent-process-only mode (bug 1785106 and bug 1785266).
• filter targets specific to the current tab (bug 1772310)

It would have been easier to re-use sessionContext and integrate toolbox scope in it...
But it would make sessionContext become mutable whereas it is static today.
It is built once on WatcherActor construction.
I think it is best to keep this data immutable. It can simplify the overall codebase
to have some environment data that can't change.
Otherwise, making sessionContext mutable would still require the same additional
JSWindowActor essage to update it accordingly in all processes and threads.

There's a r+ patch which didn't land and no activity in this bug for 2 weeks.
:ochameau, could you have a look please?
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